lighthouse wrote:
Sounds to me like you have it set on active follow focus.
2.1) Single Area Focus Mode
The Single Area AF, also known as AF-S in the Nikon world or One shot AF in the Canon world is a pretty straightforward way to acquire focus. You pick one focus point and your camera will look for contrast just in that single focus point. When you half-press the shutter or press a dedicated AF button (if you have one), the camera will snap into focus once and if your subject moves, it wont reacquire focus even if you continue half-pressing the button. Hence, the focus remains locked. The Single Area AF mode often requires the camera to lock into focus before allowing you to take a picture, so if focus is not acquired or your subject moves, pressing the shutter will do nothing (due to focus error). Some cameras allow you to change this behavior though on the Nikon D300s, for example, you can set the AF-S Priority Selection under Autofocus custom settings menu to Release, which would let you take pictures even when focus is not acquired properly. A couple of things to note about the AF-S mode if you mount an external speedlight that has an AF-Assist infrared light, you will need to be in AF-S mode for it to work. The same is true for the AF-Assist lamp in front of your camera, it will only function in AF-S mode.
2.2) Continuous/AI Servo Focus Mode
Another focus mode that is available on all modern DSLRs is called Continuous/AF-C (Nikon) or AI Servo (Canon). This mode is used for tracking moving subjects and it is a must for shooting sports, wildlife and other non-stationary subjects. The way this mode works, is it analyzes the subject movement and predicts where the subject will be, placing the focus at the predicted point. The nice thing about the Continuous mode, is that it will automatically readjust focus if you or the subject move. All you need to do is continue half-pressing the shutter button or holding the dedicated AF button (if you have one) on your camera and the autofocus system will automatically track any movement. Compared to Single Area AF, the Continuous mode is generally highly configurable (especially on high-end DSLR models) and can do complicated tasks, such as tracking subjects with a single or multiple focus points.
2.3) Single/Continuous Hybrid Mode
Some cameras also have another mode called AF-A (Nikon) or AI Focus AF (Canon), which is basically a hybrid mode that automatically switches between Single/One-Shot and Continuous/AI Servo modes. The way this works, is the camera detects if the subject is stationary, in which case it automatically switches to Single focus, while if the subject moves, it will switch to Continuous focus. The default method on lower-end Nikon DSLRs is AF-A and it works quite well for most situations. Many of the higher-end/professional DSLRs do not have this mode, since it is designed for beginners.
2.4) Full-time Servo Focus Mode
The new Full-time Servo AF mode, also known as AF-F is a brand new mode that was introduced by Nikon to the latest Nikon DSLRs like Nikon D3100 and Nikon D7000, specifically for recording video in Live View mode. This mode automatically tracks subject movement and acquires focus during video recording. While it seems to sound like a great feature, it does not work very well for fast-moving subjects and Nikon will have to work on improving this mode to make it faster and more usable. Dont worry about this mode if you do not shoot video.
Read more: http://photographylife.com/dslr-autofocus-modes-explained#ixzz2yo3NnLyF
Sounds to me like you have it set on active follow... (
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I normally have camera set to AF-S or rarely AF-A. All the photos where the focus point apparently moved were on still subjects like portraits or landscape/scenery.